In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

The names of many genes reflect the phenotype (physical appearance) that results when the gene is mutated instead of its "normal" phenotype. This is because scientists often discover what a gene does only when a mutation—introduced by the scientists or occurring at random—produces a striking change in appearance. For example, mutations in the small eye gene of mice cause the animals to have smaller eyes than normal or no eyes at all. Names can also reflect pop culture sensibilities: The Sonic hedgehog gene was named after a popular video-game character when scientists discovered that mutations gave developing embryos the appearance of a hedgehog.